An aluminum electrolytic capacitor generally has the following configuration. Namely, a stripe-shaped high-purity aluminum foil is subjected to a chemical or electrochemical etching process for increasing a surface-area thereof. The surface-area-increased aluminum foil is then subjected to a conversion treatment in a conversion solution such as an ammonium borate solution, so as to form an oxide film on a surface of the aluminum foil, thereby forming an anode foil. A cathode foil is similarly formed of a surface-area-increased high-purity aluminum foil. The anode foil and the cathode foil are laminated through a separator. This lamination structure is further rolled to form a capacitor element. This capacitor element is further impregnated with an electrolytic solution and then contained in a cylinder-shaped bottom-closed metal outer case. Further, a scaling member made of elastic rubber is also contained from the opening edge of the outer case. Furthermore, the opening edge of the outer case is scaled by a closing process to form an aluminum electrolytic capacitor.
As electrolytic solutions to be impregnated into the capacitor element of a small and low-pressure aluminum electrolytic capacitor, there have been conventionally known the electrolytic solution comprising a main solvent of ethylene glycol and a solute of an ammonium salt such as an adipic acid and a benzoic acid, and other electrolytic solutions comprising another main solvent of γ-butyrolactone and another solute of a quaternary cyclic amidinium salt such as a phthalic acid and a maleic acid.
By the way, electronic information devices have been digitalized in recent years, and the increase in driving frequency of microprocessor units (MPU) as a core part of these electronic information devices has been in progress, resulting in an increase in electric power consumption, and raising a remarkable problem of reliability caused by heat generation. As a countermeasure against this, attempts to reduce the driving voltage have been made. As a circuit for supplying a highly accurate electric power to the microprocessor, a DC-DC converter so called as a voltage regulator module (VRM) has widely been used. For an output-side capacitor, a large number of capacitors with a low equivalent series resistance (ESR) are used for preventing any voltage drop. As a capacitor having this low ESR characteristic, a solid electrolytic capacitor using a solid electrolyte has been practiced in use and widely used as a capacitor suitable for these purposes.
The increase in driving frequency of the microprocessor has been remarkable, however, with increasing the power consumption. In order to respond to that, the increase of the power supplied from the capacitor has been requested for preventing any voltage drop. In other words, a large power supply must be made in a short time, for which purpose the above-mentioned solid electrolytic capacitor is needed to not only be increased in capacity and decreased in size and voltage but also have the ESR characteristic lower than ever.
Attempts have been made to further reduce the resistivity of the electrolytic solution by including a large amount of water in the electrolytic solution. Despite the low resistivity of the electrolyte, however, such electrolytic capacitor has other problems with an insufficient effect of reducing the ESR thereof, and a non-good aging property.
As described above, there is a limitation to the reduction of the ESR of the capacitor by improving the electrolytic solution to be used for electrolytic capacitor, and a further reduction of the ESR remains as a difficult problem.
The present invention was made to solve the above-described problem and provides an electrolytic capacitor with a realized low-ESR and an electrode foil to be used for electrolytic capacitor